1. Field of the Invention
The present disclosure generally relates to methods of controlling active material actuators, and more particularly, to a method of controlling and/or predicting the remaining useful life of an active material actuator, such as a shape memory alloy actuator, utilizing an operational envelope developed based on an inherent system variable, such as resistance, over a secondary variable, such as time (e.g., the change in resistance inherent to the actuator over an actuation cycle).
2. Discussion of Prior Art
Among active material actuators, shape memory alloy (SMA) actuators, in the Martensite phase, are activated by heating the SMA material to a temperature that is above a prescribed value. This causes the material to undergo phase transformation from the Martensite to the Austenite phase, wherein it contracts and in the process provides linear or angular displacement. A common method of activation involves resistively heating the SMA by applying an electrical current therethrough. Concerns with using SMA actuators continue to include overheating, i.e., applying an excess of heat energy above what is required to actuate the wire, and overloading, i.e., applying an excessive stress load, for example, by blocking the output. Overheating and overloading can cause longer cooling times, reduced system response bandwidth, and in some cases damage to the wire. It is therefore desirable to have an effective and robust means of controlling wire actuation to prevent overheating and overloading, to provide consistent output and streamlined actuation over the life of the actuator, and to accurately predict the remaining useful life of the actuator.
Traditionally, various external sensors and/or mechanical devices, such as temperature and position sensors, have been used to alleviate concerns relating to overheating, overloading, and variation/degradation in output. However, these provisions add to the complexity, costs, and packaging requirements of conventional actuators. Closed-loop controls have been developed that monitor absolute actuator resistance to detect, among other things, start of actuation, end of actuation, overload, and/or reset or ready for next actuation cycle state. These methods, however, present their own limitations. For example, hysteresis of the resistance, relatively small (5-10%) change in electrical resistivity, small value of intrinsic resistivity, and external factors, such as noise and ambient conditions, have all impacted the reliability of these approaches.